It’s her trademark appearance! The character of Isobel “Izzie” Stevens.

A peak on her underwear revealed a trademark quote by Derek Shepherd  in the series which was:“Well, this is fun. Hello again. I like the Hello Kitty, by the way.” There are several other references made to her wearing it, but I have not got a picture of it.

I wonder if this was an official endorsement sponsored or slipped in just for fun?